Whether a single instance or clustered database, Oracle tracks (logs) all changes to database blocks in
online redolog files. In an Oracle RAC environment, each instance will have its own set of online
redolog files known as a thread. Each Oracle instance will use its set (group) of online redologs
in a circular manner. Once an online redolog fills, Oracle moves to the next one. If the database is in
"Archive Log Mode", Oracle will make a copy of the online redo log before it gets reused. A thread must
contain at least two online redologs (or online redolog groups). The same holds true for a single instance
configuration. The single instance must contain at least two online redologs (or online redolog groups).

The size of an online redolog file is completely independent of another intances' redolog size. Although
in most configurations the size is the same, it may be different depending on the workload and
backup / recovery considerations for each node. It is also worth mentioning that each instance has exclusive
write access to its own online redolog files. In a correctly configured RAC environment, however, each
instance can read another instance's current online redolog file to perform instance recovery if that
instance was terminated abnormally. It is therefore a requirement that online redo logs be located on a
shared storage device (just like the database files).

As already mentioned, Oracle writes to its online redolog files in a circular manner. When the current
online redolog fills, Oracle will switch to the next one. To facilitate media recovery, Oracle allows
the DBA to put the database into "Archive Log Mode" which makes a copy of the online redolog after it
fills (and before it gets reused). This is a process known as archiving.

The Database Creation Assistant (DBCA) allows users to configure a new database to be in archive log
mode, however most DBA's opt to bypass this option. In cases like this where the database is in no archive
log mode, it is a simple task to put the database into archive log mode. Note however that this will require
a short database outage. From one of the nodes in the Oracle RAC 11g configuration, use the following
tasks to put a RAC enabled database into archive log mode. For the purpose of this article, I will use
the node racnode1 which runs the racdb1 instance:

Login to one of the nodes (i.e. racnode1) and disable the cluster instance parameter
by setting cluster_database to FALSE from the current instance:

After enabling Archive Log Mode, each instance in the RAC configuration can automatically
archive redologs!

About the Author

Jeffrey Hunter is an Oracle Certified Professional, Java Development Certified Professional, Author,
and an Oracle ACE.
Jeff currently works as a Senior Database Administrator for
The DBA Zone, Inc. located in Pittsburgh, Pennsylvania.
His work includes advanced performance tuning, Java and PL/SQL programming, developing
high availability solutions, capacity
planning, database security, and physical / logical database design in a UNIX /
Linux server environment. Jeff's other interests include mathematical
encryption theory, tutoring advanced mathematics, programming language processors (compilers and interpreters)
in Java and C, LDAP, writing web-based database administration tools, and of
course Linux. He has been a Sr. Database Administrator and Software Engineer
for over 20 years and maintains his own website site at:
http://www.iDevelopment.info.
Jeff graduated from Stanislaus State University in Turlock,
California, with a Bachelor's degree in Computer Science and Mathematics.

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